Commercial airplanes and components thereof are increasingly being manufactured from composite materials that may require ultrasonic inspection to determine part integrity. In order to perform an ultrasonic inspection, a transducer that transmits and/or receives sound waves must be acoustically coupled to the part. Water or some other liquid is typically disposed between the transducer and the part to be inspected to provide a medium through which sound waves can travel.
Inspection may be performed from one side (i.e., pulse-echo inspections) or from two sides, referred to herein as the through-transmission (TTU) method. The one side, pulse-echo inspection method is not effective on honeycomb core sandwich panels, so the TTU method is needed to inspect such structures. In addition, the TTU method may provide a more detailed assessment of the condition of a part than other methods. For example, TTU inspection can be used to quantify the amount of porosity present in a composite laminate. A TTU inspection is performed by transmitting sound waves into one side of the part and receiving the sound waves on the opposite side of the part using two transducers coupled to the part simultaneously and on opposite sides.
Challenges may arise when a part to be inspected by the TTU method has only one smooth simple surface, with its opposite surface having complex curvatures and/or out-of-plane features. The transducer coupled to the complex surface must be maintained in angular alignment with the transducer on the simple surface, which may require it to be oriented in a way that deviates from the local surface contour on the complex side. Because of these challenges, some prior art TTU methods require immersing the entire part to be inspected in a tank of coupling fluid, with the transducers positioned far enough from the complex surface to avoid interference with out-of-plane features, while still maintaining acoustic coupling. This solution is inefficient and not practical for large parts.
Alternatively, a squirter system can enable TTU inspection by sending sound waves through moving streams of liquid directed at opposing sides of a part. This approach requires a large amount of liquid to be pumped and recovered, particularly if the transducer is large or an array of transducers is used, as is often the case with inspection of aircraft components. Furthermore, the moving stream of liquid creates splashing that can interfere with transmission of the sound waves.